In most of the cases, the pieces produced by powder metallurgy need to be calibrated after the sintering step due to the dimensional variations that occur during sintering. Lubricant oil is used in the calibration to reduce friction and wear of the machine tools, as well as to facilitate extraction of the pieces from the calibration matrix. Oil is likewise used for storing sintered pieces and pieces produced by other manufacturing technique. For example, refrigerant oil is used for machining high precision pieces.
Aiming at improving the properties of the finished pieces, such as wear resistance, corrosion resistance and fatigue resistance, there are often used surface thermochemical treatments, such as nitration, cementation, carbonitration, etc. In order to effect these thermochemical treatments, the presence of oil on the surface and in the pores of the pieces is prejudicial, especially when the thermochemical processing is effected via plasma. For example, during plasma nitration, the oil retained in the pores and on the surface of the pieces produces instabilities in the electrical discharge, contamination of the reactor, inadequate formation of the superficial layers formed (for example, nitrates) and contamination with carbon of the material submitted to treatment by means of an inefficient cleaning. Thus, the oil must be completely removed before the thermochemical treatments of surface hardening.
Conventionally, a chemical cleaning is carried out in ultrasound with organic solvents (for example hexane, petroleum ether or alcohol) further followed by a heat treatment in atmosphere containing hydrogen or oxygen in an industrial electric oven, aiming at eliminating completely all the organic residues from the pieces. When communicating residual pores are present, which generally occurs in sintered steels, the cleaning is especially difficult, besides being pollutant due to the pollutant products used.
In some known treating methods, the operations of cleaning and thermochemically treating the surfaces are carried out in two separate steps in distinct equipment, which requires a very long processing time, typically 20 hours, leading to low productivity and high cost.
With the purpose of obtaining a complete removal of the oil and other organic and inorganic contaminants from the surface or pores of the metallic pieces, and also simplifying and abbreviating a subsequent surface thermochemical treatment operation of said pieces in the same thermal cycle, there has been proposed the process of cleaning and surface treatment object of Brazilian patent application PI-0105593-3, of the same applicant, according to which the pieces to be cleaned are positioned on a support provided inside the plasma reactor and connected to an anode of the latter, the cathode of said reactor being connected to a negative potential. The assembly defined by the support and pieces is surrounded by an ionized gas at low pressure and containing ions, neutral atoms and electrons, known as plasma and which is generated by an abnormal electrical discharge. The electrons provoke an electronic bombardment on the assembly defined by the support and pieces and connected to the reactor anode.
The generation of gaseous plasma in the interior of the reactor allows the plasma reactive environment formed around the pieces to be used to catalyze the reaction of dissociating the molecules of the oil and of other possible contaminants existing in the pieces, allowing the vaporization of said contaminants and the complete elimination thereof through exhaustion, under vacuum, from the inside of the reactor. The heat generated by the plasma, by the collision of fast ions and neutral atoms against the cathode, is usually sufficient to provide vaporization of the molecularly dissociated oil, without requiring relevant changes in the plasma parameters more adequate to catalyze the reactions of interest in each cleaning operation.
However, in plasma reactors of certain dimensions (and in certain chemical reactions of molecular dissociation of the contaminants), it may occur that different inner regions of the reactor remain at a temperature which is sufficiently low to allow condensation of the contaminant vapors prior to the dissociation and progressive deposition thereof in these relatively cold inner regions of the plasma reactor, contaminating the system with carbon-based compounds that are harmful to the subsequent surface treatments.
Moreover, in many surface thermochemical treatment operations occurring subsequently to the cleaning operation and carried out inside the same reactor, the heat generated by the plasma is not enough to maintain the heating rate and the process temperature required to obtain the desired surface treatment.
Thus, even if the surface treatment operation has been executed, as disclosed in said prior patent application, by reverting the electrical circuit between the anode and the cathode, and also by surrounding the pieces with gaseous plasma of ions with high kinetic energy, and by applying an electrical discharge to the cathode so as to provoke an ionic bombardment on the pieces, there is a need to adjust the plasma parameters, not as a function of the reactions of interest, but aiming to obtain temperature levels inside the plasma reactor that are sufficient and necessary for the desired surface treatment. In this case, the temperature variations required inside the reactor are obtained as a function of the electrical discharge parameters, but some situations may exist in which the intensity of the electrical discharge required for the production of determined temperatures leads to the formation of electrical arcs in the reaction environment, causing superficial damages (marks on the pieces) and contamination by carbon deposits on the surfaces of the pieces, impairing the subsequent thermochemical treatments, besides the fact that the thermal gradient negatively influences the formation and homogeneity of the formed layer.
The provision of a resistive heating in plasma reactors is known in the art.
In one of these known reactors, there is provided an external resistive heating for removing the binders and possible contaminants from the pieces obtained by sintering. However, in this known construction, the pieces to be submitted to a treatment for removing binders and contaminants are applied to the reactor cathode, leading to the formation of electric arcs and consequent contamination of the pieces with carbon, which is harmful to the subsequent surface treatments.
In another known type of reactor disclosed in patent U.S. Pat. No. 6,579,493, there is provided an inner resistive heating to obtain high temperatures sufficient to remove the binders and certain contaminants from the metallic pieces obtained by powder metallurgy and also to provide sintering of the pieces. Nevertheless, the provision of resistive heating in the interior of this type of reactor requires the use of high cost materials, such as molybdenum and the provision of heat radiation reflecting elements between the inner resistance and the reactor wall, and also the provision of cooling in said outer wall. This solution is inadequate for cleaning the organic contaminants from the pieces under treatment, since it allows the volatile vapors of oils and other contaminants to be condensed and deposited on the cold regions of the heat radiation reflecting elements and on the reactor wall, before they are exhausted from the reaction environment, contaminating the latter and the pieces contained therein with carbon-based compounds that impair the subsequent surface thermochemical treatments of the pieces.
From the above, there is a need for the provision of a solution which allows obtaining, in the interior of the reactor, homogeneous and even high temperatures as a function of the desired surface treatment, in a way that is independent from the electrical discharge parameters that are more adequate for catalyzing the reactions of interest in each case.